A glass optical transmission medium having a region in which refractive index varies from the center of linear body or bar-like body toward the outer periphery has been already proposed and put into practical use. In particular, the glass optical transmission medium is used as an optical transmission medium having a refractive index distribution capable of providing the bandwidth characteristics required for the application to the optical communication.
However, the glass optical transmission medium involved a problem in its mechanical properties such as poor flexibility and poor shock resistance. It also has some other drawbacks such as low productivity and high production costs.
In contrast with the distributed-refractive-index type glass optical transmission medium, methods of manufacturing distributed-refractive-index optical transmission medium have been proposed in Japanese Patent Laid-open Publication Nos. 3-42603, 3-64704, 3-65904, 3-81701, 3-91703, 3-81704 and 3-81706.
These manufacturing methods, however, had various drawbacks. For example, in the manufacturing method of the optical transmission medium using the difference between the reaction ratios of monomers constituting a polymer, it was impossible to manufacture the optical transmission medium having an elongated transmission distance because the manufacturing method is comprised of a batch process.
On the other hand, the method of manufacturing the optical transmission medium using a melt spinning method exhibited an inconvenience that the control of the distribution of refractive indices is difficult to perform since the materials conferring a transparency are not to be selected due to its copolymer method using two or more types of monomers having reactivity.
Furthermore, in Japanese Patent Application No. 3-27345, the present inventor proposed a manufacturing method capable of providing an excellent transparency even to distributed index optical transmission medium. Unlike the case of the conventional manufacturing method by polymerizing the monomers, in this method, non-polymerizing materials are used in order to suppress the optical scattering due to the phase separation caused by the difference in the polymerizing speeds, thereby reducing the optical guiding loss.
However, this method also has disadvantages such as low productivity and lack of ability to uniformly manufacture elongated optical transmission bodies due to its being dependent on the batch process.